In recent years, the degree of integration of semiconductor integrated circuits and other microelectronic devices has continued to increase, with accompanying decreases in the “critical dimensions” (minimum feature size) of such devices. Currently, optical “steppers” are the workhorse microlithography systems used for manufacturing microelectronic devices. “Optical” microlithography is performed using a beam of ultraviolet light that illuminates a reticle defining the pattern to be transferred to the substrate. Light of the beam passing through the reticle is projected onto a resist-coated upstream-facing surface of a suitable substrate. The reticle is created normally by electron-beam “direct-mask writing.”
With the relentless drive to progressively smaller feature sizes (now less than 0.25 μm) the pattern-resolution limitations of optical microlithography have become a major limitation. To solve this problem, considerable effort currently is being expended to develop a practical “next generation” microlithography technology. A major effort to such end involves using a charged particle beam (e.g., an electron beam) as the lithographic energy beam. Charged-particle-beam (CPB) microlithography is expected to produce substantially better pattern resolution for reasons similar to the reasons for which electron microscopy yields better image resolution than optical microscopy.
The first type of CPB microlithography that received serious attention was electron-beam direct writing on the lithographic substrate, in a manner similar to the technique that had been used for manufacturing reticles for use in optical microlithography. Because the pattern is drawn on the substrate feature-by-feature, the major drawback of electron-beam direct-writing is that throughput is extremely low. Consequently, various approaches have been considered for increasing the throughput of CPB microlithography. An approach currently being used to a limited extent involves any of the several “partial pattern block” exposure methods such as “cell projection,” “character projection,” and “block exposure.” In the partial pattern block exposure methods, a very small portion (e.g., 5 μm square) of a circuit pattern (the portion typically being a highly repeated portion of the overall pattern, such as a single DRAM memory cell) is defined on a mask and repeatedly exposed onto respective regions of the substrate. Whereas partial pattern block exposure methods exhibit better throughput than direct-writing methods, the throughput still is too low for most mass-production purposes.
Furthermore, the non-repeated portions of the overall pattern must be exposed using a different technique, such as a “variable-shaped beam” method, which reduces overall throughput and increases the complexity of the lithographic process.
An approach that offers substantially improved throughput compared to partial pattern block exposure is “reduction” pattern transfer performed using a segmented reticle. (This technique also is termed “divided-reticle” pattern transfer.) In this approach, the entire pattern is defined on the reticle. However, since it currently is impossible to projection-expose the entire pattern in one exposure “shot” onto the substrate (see below), the pattern as defined on the reticle is divided into multiple “subfields” each defining a respective portion of the overall pattern. The subfields are exposed in respective shots from the reticle to the substrate. Exposure of an individual subfield involves projection of an image of the subfield using a projection “lens” that “reduces” (demagnifies) the subfield images as projected onto the substrate. The individual subfield images are formed on the substrate in respective locations serving to “stitch” the images together in a contiguous manner to form the entire projected pattern on the substrate after completing exposure of all the subfields.
As noted above, it currently is impossible to projection-expose an entire pattern using a charged particle beam. The principal reasons are as follows. (1) It currently is impossible to configure illumination- and projection-optical systems capable of projecting an entire die pattern in one shot without introducing excessive aberrations especially off-axis. (2) It currently is impossible to fabricate a reticle having sufficient size to define an entire pattern for one-shot exposure. Hence, the divided-reticle pattern-transfer technique offers the currently best prospects for achieving good lithographic resolution at an acceptable throughput.
Another advantage of the divided-reticle pattern-transfer method is that it allows certain corrective compensations to be performed in real time as each subfield is being exposed, thereby significantly improving pattern-transfer accuracy. These compensations include aberration corrections, image-magnification corrections, and image-position corrections of the illumination- and projection-optical systems of the microlithography apparatus. To expose each subfield, the charged particle beam is laterally deflected as required. In addition, the reticle and substrate are moved relative to each other to obtain proper positioning for exposure.
In the “cell projection” method summarized above, a so-called “absorption-stencil” mask is utilized, in which a minute pattern of through-holes is formed in a thin membrane of silicon. The through-holes correspond to respective pattern features, and the intervening portions of the membrane correspond to non-patterned regions. The membrane is relatively thick, approximately 20 μm. As an illumination beam impinges on such a mask, charged particles incident on the membrane are absorbed by the membrane, and charged particles incident on a through-hole pass unobstructed through the hole. Thus, the portion of the illumination beam transmitted through the mask is “patterned” by the mask (i.e., the beam acquires an aerial image of the pattern portion defined by the illuminated portion of the mask).
In a similar manner, absorption-stencil reticles are used in divided-reticle pattern-transfer methods. However, to improve throughput, the current trend is toward increasingly higher beam currents. Absorption of incident charged particles by a reticle membrane causes heating of the membrane, which can cause deformation of the membrane and hence of the pattern. As beam current is increased, this reticle heating becomes an increasingly serious problem. In many instances, absorption-stencil reticles are no longer practical because of an inability to control the excessive thermal expansion (and deformation) of the reticle.
As a result, “scattering-stencil” reticles have been investigated. With a scattering-stencil reticle, the pattern is defined as respective through-holes in a beam-scattering membrane. As a beam is incident on such a reticle, charged particles incident on the membrane are transmitted through the membrane, but with scattering. Charged particles incident on a through-hole simply pass through without any scattering. With such a reticle, the projection-optical system of the microlithography apparatus includes a “contrast aperture” (“scattering aperture”) that blocks (by absorption) charged particles that were scattered during passage through the upstream reticle. Thus, the scattered charged particles are prevented from propagating to the downstream substrate. So as to block these scattered charged particles, the scattering aperture is situated in a beam-convergence plane of the projection lens, which is at the vicinity of the Fourier plane of the reticle. Non-scattered charged particles propagate through the scattering aperture to the substrate on which an image is formed.
In any type of stencil reticle, it is impossible to define certain pattern features such as “island” features having a membrane island surrounded by a through-hole. These features cannot be defined because the portion of the membrane forming the island is unsupported and simply falls away from the reticle. This problem is termed the “donut-pattern” problem. To solve the donut-pattern problem while using a stencil reticle, the through-hole surrounding the island must be divided between two separate pattern regions (subfields) on the reticle. The subfields (termed “complementary” subfields) are individually exposed at the same location on the substrate such that the respective through-hole portions are “stitched” together on the substrate and the island is properly situated within the pattern portion defined by the through-holes. This technique is termed “complementary pattern division.” A disadvantage of complementary pattern division is that two exposures must be performed to form a complete pattern portion at a single transferred subfield on the substrate. This need to perform double exposures reduces throughput commensurately.
In view of the problems summarized above, “scattering-membrane” reticles have been proposed. This type of reticle comprises a thin charged-particle scattering membrane on which pattern features are defined by corresponding “scattering bodies” rather than by through-holes. The scattering bodies are formed from a correspondingly patterned “high-scatterer” film formed on the surface of the reticle membrane. The membrane is transmissive to incident charged particles, but particles passing only through the membrane experience minimal scattering. Charged particles incident on a scattering body also pass through the reticle but with high scattering. A scattering-membrane reticle does not have the donut-pattern problem because the membrane has no voids. Hence, using a scattering-membrane reticle can provide better throughput than using a stencil reticle.
An example of a CPB microlithography apparatus employing a scattering-membrane reticle is described in Japan Kôkai Patent Document No. Hei 8-34169 (1996). In this apparatus, a diaphragm defining a small axial aperture is provided at the beam-convergence plane of the projection lens. With such an aperture, almost all of the charged particles that are scattered during transmission through the reticle are blocked. This causes the temperature of the diaphragm to increase substantially during use due to the absorbed kinetic energy of charged particles colliding with the material of the diaphragm. As the diaphragm temperature increases, it undergoes a corresponding thermal expansion, which changes the amount of charged-particle irradiation (that has passed through the relatively non-scattering portions of the reticle) incident on the substrate. Also, a substantial current of charged particles is incident in the vicinity of the small axial aperture. This beam current causes rapid formation of contaminant deposits (“beam-induced contamination”) that can cause any of various exposure faults at the substrate.